1. Field of the Invention:
This invention relates to phosphorous-containing organic binder compounds for refractory ceramics, a process for the preparation of suitable binder compounds, pre-mixed, ready-to-use refractory/binder materials having long shelve-lives, a method of using the pre-mixed refractory materials, and high strength refractory articles produced from the pre-mixed refractory materials.
2. Description of the Prior Art:
It is well known in the art that there are two general groups of refractory materials: Non-Basic Refractories, such as alumino silicates; and Basic Refractories, such as periclase, dead burned dolomite and magnesite, all of which are composed mainly of CaO and/or MgO.
Various materials have been used as low temperature (e.g., 400.degree.-2000.degree. F.) chemical binders in basic refractory products to impart a bond at temperatures too low for a direct bond to form through sintering. Good examples of these are sodium silicate, chromic acid and various inorganic phosphorous compounds. These compounds must be dissolved in a liquid medium to work effectively. For practical purposes, the liquid medium must be water since these binder materials are generally insoluble in non-aqueous media (e.g., organic solvents). In addition, most non-aqueous media also have other drawbacks such as flammability, toxicity and high price. These binder materials have been useful only in basic refractory products which are prepared on-site and used immediately. Water is used as the liquid media but it is in only brief contact with the basic aggregate since heat is applied to the mixture immediately after it is implaced to remove the water. Examples of such products are gunnables, rammables and castables that are sold in a dry form. Since such products are mixed with water on-site and used immediately, they are not required to exhibit a significant shelf life in the mixed or wet condition.
Premixed, ready-to-use, basic refractory ramming/patching mixes having a chemical binder uniformly dispersed throughout the mixes via a liquid phase have not been successfully produced in the past. This is due to the fact that basic refractory aggregates are not stable over long periods of time in contact with water. Both MgO and CaO will react with water to form Mg(OH).sub.2 and Ca(OH).sub.2, respectively. These reactions present the following problems with a prewetted refractory mix:
1. Volume expansion related to hydration--disrupts refractory structure--degrades particles. PA1 2. Hydration decreases free water content--bond begins to set prematurely. PA1 3. Explosion hazard where hydrates are decomposed at high temperature. PA1 (1) the binder must be substantially non-reactive with refractory aggregates, especially basic refractory aggregates, over long periods of time at ambient temperatures (normal storage periods of refractory aggregate mixtures typically range from one week to two years); PA1 (2) the binder must exist in a liquid phase or a solid phase soluble in an organic solvent; PA1 (3) the binder upon heating must decompose, before boiling, to form reactive phosphorous containing species.
Basic refractory materials cannot be premixed in an aqueous system and be expected to exhibit any shelf life. Basic refractories mixed in an aqueous system must be prepared on-site and used immediately to avoid the problems associated with hydration. In such on-site applications, the water is utilized only as the media for the implacement of the refractory. Heat is immediately applied to the mixture to remove the water.
U.S. Pat. No. 4,473,654 discloses the use of Li compounds in combination with organic materials such as drying oils to produce a premixed basic refractory with long shelf life. However, this patent does not depend upon phosphorous bonding between refractory aggregates, and further, the binder material requires at least 5% free CaO in order to work properly.
It has also been known to use organic materials such as phenolic resins and pitch to create a bond between basic refractory aggregates. However, phenolics and pitch have limited use since the bond depends on carbon which, under oxidizing conditions, burns out, thus causing the refractory article to lose its strength.
The use of inorganic phosphorous-containing compounds as basic refractory chemical binders is well known in the art; for example, aqueous solutions of sodium or potassium phosphates. Although such binders form strong chemical bonds with basic refractory aggregates, for practical purposes, such compounds are soluble only in water and have the drawbacks discussed above. To be effective, they must be dissolved in water. Therefore, any basic refractory/inorganic phosphate mixture must be prepared "on-site" and immediately used, thus displaying essentially no shelf life.
Japanese Patent No. 49-10,967 discloses the use of acidic phosphite binder materials for use with refractory ceramics. These binder materials form a suitable ceramic bond between non-basic refractory ceramics. However, because these binder materials contain acidic hydrogen, chemical reactions occur when these binder materials are used with basic refractory ceramics, and thus the refractory/binder mixture has essentially no shelf life. Furthermore, the phosphites disclosed in this patent tend to have a high vapor pressure, thus, it is likely that this material will evaporate before decomposing to form a phosphorous bond. Although this patent discloses the use of phosphite salts, it is believed that such salts are generally soluble only in water, thus prohibiting extended exposure of these materials to basic refractories, because of the hydration problems discussed above.